LiBH4/Li3AlH6 hydrogen storage composite was prepared by ball-milling, and its hydrogen storage properties and mechanism were studied by TG/DSC/MS and XRD analysis. The activation energies of the dehydrogenation reactions were calculated. The experimental results show that LiBH4-Li3AlH6 composite dehydrogenates shows three reactions: (1) Li3AlH6 decomposes with hydrogen desorption and formation of Al; (2) part of Al reacts with LBH4 with hydrogen desorption; (3) Al reacts with LiH. The total hydrogen desorption capacity reaches 8.5 wt%. And the mechanism of hydrogen desorption process of LiBH4-Li3AlH6 composite was discussed